Former President Bill Clinton, underscoring his wife's determination against statistically long odds of overcoming Mr Obama's pledged delegate lead, said in California on Sunday a 'vigorous debate' was good for the party and those who want to see the former first lady quit the contest should just 'chill out.' 'We're going to win this election if we just chill out and let everybody have their say,' Mrs Clinton said.

A crowd of about 20,000 was chilled as supporters waited in cool temperatures for Mr Obama to speak on the campus lawn at Penn State University, where the Illinois senator agreed that the tough campaign was healthy if frustrating.

'As this primary has gone on a little bit long, there have been people who've been voicing some frustration,' Mr Obama said.

'I want everybody to understand that this has been a great contest, great for America. It's engaged and involved people like never before. I think it's terrific that Senator Clinton's supporters have been as passionate as my supporters have been because that makes the people invested and engaged in this process, and I am absolutely confident that when this primary season is over Democrats will be united.'

After Vermont Sen. Patrick Leahy last week became the first leading Democrat to openly call on Mrs Clinton to abandon her bid and back Mr Obama, the front-runner said Mrs Clinton should stay in the race as long as she wanted.

He also said he had not talked to Mr Leahy before he issued his statement on Mrs Clinton leaving the race. But Mr Leahy's sentiment is shared by many activists worried that a drawn-out nominating contest only bolsters Republican nominee-in-waiting John McCain.

Other Obama supporters have echoed that view while stopping short of asking Mrs Clinton to withdraw.

Obama's lead insurmountable
New Mexico Gov. Bill Richardson on Sunday called Mr Obama's lead all but insurmountable, while Massachusetts Sen. John Kerry said the contest would be reaching 'a point of judgment' very soon.

'I don't think it's up to our campaign or any individual to tell Hillary Clinton or their campaign when that is,' Mr Kerry, the 2004 Democratic nominee, said on ABC's 'This Week' on Sunday. 'But there will be, I think, a consensus about it, and I think it's going to occur over these next weeks.'

As she has fought through three-months of primary voting, Mrs Clinton has been able count on a deep well of support from women voters, many of whom see the attempt to push her out of the race now as the work of a male-dominated party structure.

In California, Mrs Clinton beat Obama by a margin of 59 per cent to 36 per cent among women. She bettered him by 54 per cent to 45 per cent among women in Ohio, an important general election battleground state.

Mr Obama, in turn, has walloped Clinton among men in nearly every state. He has prevailed among women in just a handful of places, including his home state of Illinois and states with large black populations.

As Mr Obama crossed Pennsylvania, which votes on April 22, he pounded his message of reform and said again that Mrs McCain was running for US President George W. Bush's third term.

Mr McCain, the Arizona senator and Vietnam war hero, gave a major foreign policy speech last week trying to blunt Democratic attempts to paint him as a candidate who would view the world through Mr Bush's pre-emptive foreign policy lens.

'Our great power does not mean we can do whatever we want whenever we want, nor should we assume we have all the wisdom and knowledge necessary to succeed,' said in the carefully honed speech to the Los Angeles World Affairs Council. 'We need to listen to the views and respect the collective will of our democratic allies.'

Clinton vows to continue her campaign
Nevertheless, Mr McCain has not backed away from his support for the unpopular Iraq war and has, in fact, said his candidacy would likely stand or fall on the outcome. Violence has flared anew in Iraq despite the US troop surge that Mr McCain said has been a success.

Mrs Clinton, in vowing to continue her campaign until August if necessary, returned to the open Democratic Party sore of the negated primary votes in Michigan and Florida.

She won both contests, but the national party had said in advance the contests would not count because the state parties ignored the prohibition against holding the votes too early. Mr Obama had taken his name off the Michigan ballot for that reason and neither candidate focused campaigning in those states.

But Mrs Clinton trails Mr Obama by 1,624 to 1,499 in national delegates, including both those pledged as a result of state primaries and caucuses as well as superdelegates - elected and party officials who can vote for whomever they wish.

Mr Obama picked up one more superdelegate endorsement Sunday, from Minnesota Sen. Amy Klobuchar.

Mrs Clinton now insists the Michigan and Florida votes be added to her tally. She won in both states.

'I know there are some people who want to shut this down and I think they are wrong,' Mrs Clinton told the Washington Post in an interview published Sunday.

'I have no intention of stopping until we finish what we started and until we see what happens in the next 10 contests and until we resolve Florida and Michigan. And if we don't resolve it, we'll resolve it at the convention - that's what credentials committees are for,' she said, referring to what many fear could be a divisive fight on the floor of the party convention in Denver, from Aug. 25-28. -- AP